Many laundry detergents contain boron-containing compounds such as boric acid, sodium borate or sodium perborate. Sodium borate, also known as borax, is used as a builder or calcium sequestrant, improving detergency properties in hard water. Borate buffers the detergent at around pH 9-11. It also acts to stabilize pigment soil and stabilize certain enzymes such as proteases and amylases. Sodium perborate, in either the monohydrate or tetrahydrate form, is added in some detergents as a peroxygen bleach. Either alone or in combination with a bleach activator such as TAED or NOBS, perborate generates hydrogen peroxide in situ when diluted into the water of a washing machine, and the hydrogen peroxide is effective in bleaching certain oxidizable stains such as protein-based stains.
Enzymes are useful additives to laundry detergents for their efficacy in hydrolyzing and removing many different types of stains. For example, proteases, amylases and lipases remove stains based on protein, starch, and triglyceride oils. Some enzymes are useful for their benefits in modifying or restoring fabric properties. For example, cellulases can be used to remove frayed or pilled cellulose fibers to restore the color, texture and appearance of cotton-based fabrics. To achieve these benefits in powdered laundry detergents, the enzymes must be added in a granulated form. These granules or particles typically require a strong outer coating of low permeability to serve as a barrier during storage in the detergent against heat, humidity, and diffusible oxidants, such as peroxygen bleaches and hydrogen peroxide. Further, a tough or flexible outer coating can help to increase the mechanical strength and attrition-resistance of the enzyme granule. This is important in reducing the tendency of the granule to produce sensitizing protein dusts upon handling, for example in the production line of a detergent manufacturing plant. Sensitizing dusts have been known to induce allergic responses in detergent factory workers, and effective enzyme granule coatings are a principal means of reducing the levels of airborne enzyme dusts and aerosols in detergent factories.
Polyvinyl alcohol (PVA) has proven to be a very effective coating for detergent enzyme granules. Examples of the use of PVA in enzyme granule coatings can be found, for example, in U.S. Pat. No. 5,324,649. PVA is particularly useful because it simultaneously provides a coating with reduced permeability to moisture and oxidants, a strong and attrition-resistant coating, and a coating, which is readily soluble in water and detergent solutions in both cold and hot water. It is also sufficiently water soluble that it can readily be prepared in coating solutions, and coated onto enzyme-containing granules at reasonable rates, for example in fluidized bed spray-coaters. Such a coating process is described in aforementioned U.S. Pat. No. 5,324,649. PVA is available in a wide range of molecular weights and degrees of hydrolysis, allowing one skilled in the art to control the relative solubility and physical properties of the polymer coating, which can be optimized to balance factors such as the ease of coating, dissolution rate of the granule, attrition resistance of the granule, and permeability of the granule to moisture and oxidants. PVA is also readily plasticized with water, glycerol, triethylene glycol, polyethylene glycol, formamide, and triethanolamine acetate, and other polyhydric compounds, and is compatible with pigments and fillers such as titanium dioxide, talc, and calcium carbonate, and dyes.
One of the unfortunate properties of PVA, however, is its tendency to become crosslinked by a number of chemical species, including sodium borate, sodium perborate, aldehydes, and certain dyes (e.g., Protamine, Mobay Corp.). Borates, perborates and other boron-containing compounds form adducts with the vicinal hydroxyl groups of PVA at alkaline pH's, resulting in water-insoluble complexes or gels. This insolubility of the borate-PVA gels is reversible upon a shift towards more acidic pH. In addition, agitation or higher temperatures can also prevent the formation of an insoluble gel layer since dissolution and dilution of the PVA is more rapid than crosslinking of PVA under these conditions. Unfortunately, in many laundry applications, the presence of borate and the washing conditions result in the insolubilization of any PVA present in the coating or interior of enzyme granules. The PVA coating typically contains a pigment or filler such as titanium dioxide or talc, and once the coating is gelled or insolubilized, it remains as a visible shell or residue, which attaches to clothing due to its gummy nature when hydrated. These shells persist as visible residue on clothing, which is undesirable to consumers.
The crosslinking or gelation of PVA-coated granules frequently makes them unacceptable for use in borate- or perborate-containing detergents. To some extent, the degree of crosslinking can be modified by the addition into the coating of fillers or extenders, such as talc, clay, starch or maltodextrin. Blending PVA with other substances to create soluble films or pouches is described in U.S. Pat. No. 4,828,744 and U.S. Pat. No. 4,626,372. However, the PVA will still tend to cross-link even at levels as low as 10% w/w in the coating, and such a drastic reduction of PVA in the coating tends to obviate its barrier and mechanical strength properties. U.S. Pat. No. RE34,988 describes a modified PVA, dissolvable pouch containing enzymes; however, pouches typically do not provide uniform enzyme release.
Thus there is a need in the art for a particle coating having a vinyl polymer or copolymer composition sufficient to provide barrier and tensile strength properties without significant crosslinking or gelation of the vinyl polymer or copolymer in the presence of chemicals such as sodium borate, sodium perborate and other boron containing compounds.